Removing clutter from radar cross section measurements using spectral tagging

ABSTRACT

A system for performing radar cross section measurements of a target may include a radar system and an antenna associated with the radar system to transmit signals and to receive reflected signals from the target and a clutter source. An EM tagging device is locatable proximate to the clutter source to spectrally tag the clutter source by causing changes in an electromagnetic signal reflected by the clutter source when a predetermined radar signal transmitted by the radar system is incident on the target, the clutter source and the EM tagging device. A module may identify a spectrally tagged component of reflected signals received by the radar system from the target, the clutter source and the EM tagging device. The module monitors the spectrally tagged component, compensates for variations in an un-tagged component of the reflected signals caused by the clutter source and EM tagging device, and subtracts contamination caused by the clutter source and EM tagging device and their interactions with the target, to provide a radar cross section of the target with reflected signals from the clutter source removed.

FIELD

The present disclosure relates to radar systems and performing radarcross section measurements, electromagnetic (EM) scattering measurementsor similar measurements, and more particularly to removing clutter fromradar cross section measurements or EM scattering measurements usingspectral tagging.

BACKGROUND

Performing electromagnetic (EM) scattering measurements in the presenceof clutter sources or objects which can reflect or scatterelectromagnetic or radar test signals or fields can adversely affectsuch measurements. The scattered or reflected signals or fields from theclutter source can interact or interfere with the desired scattered orreflected signals from the target under test. Conventional methods ofvector background subtraction do not work effectively when there arelarge interactions between the target-under-test and the clutter source.Examples of such clutter sources are Target Support Systems used tomount Radar Cross Section (RCS) Targets in both indoor and outdoor RCSranges. These supports can severely contaminate the RCS of the Targetreturn, and, under certain common situations, can interact significantlywith the target-under-test. The ability to identify such target supportcontamination and other clutter sources and the interaction between suchclutter sources and the target would allow the removal of thesecontaminants and the retrieval of accurate RCS target data.

Currently known systems and methods for removing the effects of cluttersources involve hardware solutions, software solutions and a combinationof hardware and software solutions. Hardware methods generally attemptto reduce the RCS of the clutter through shaping the clutter source,selecting the materials from which the clutter source is constructed,treatment of the materials of the clutter source or some combination ofthese techniques to substantially reduce or eliminate the scattering orcontamination by the clutter source. Such hardware solutions may notcompletely eliminate the clutter contamination to desirable oracceptable levels. Even for large targets, a very small amount ofclutter contamination may create an undesirable degradation in the data.

Software solutions may generally involve combinations of vectorbackground subtraction, image editing and reconstruction and Dopplerfiltering. Software solutions typically cannot account for theinteractions between the clutter and the target. Accordingly suchsolutions may be ineffective when these interactions are significant orcommon. For example, vector background subtraction involves ameasurement of the clutter without the target, and then a measurement ofthe target in the presence of the clutter. Subtracting the latter fromthe former yields the target return and the interactions when both theclutter and target are present. Under certain conditions, theseinteractions can be as large a contaminant as the clutter alone, so theresult can still be significantly degraded.

BRIEF SUMMARY

In accordance with an embodiment, a system and method are disclosedwhich permit unique identification of contaminating signals or fieldsfrom clutter sources with the target present and without disturbingexisting fields or the target RCS. Clutter contamination signals orfields including interaction between the target and the clutter can beidentified and removed by spectral tagging of the clutter sources usingan electromagnetic (EM) tagging device or EM surface. The tagging devicemay be modulated between two or more distinct RCS states to spectrallytag the clutter source. The fields reflected from the clutter source andEM tagging device or EM surface contains both tagged and un-taggedcomponents.

In accordance with an embodiment, a system for performing radar crosssection measurements of a target may include a radar system and anantenna associated with the radar system to transmit signals and toreceive reflected signals from the target and a clutter source. An EMtagging device is locatable proximate to the clutter source tospectrally tag the clutter source by causing changes in anelectromagnetic signal reflected by the clutter source when apredetermined radar signal transmitted by the radar system is incidenton the target, the clutter source and the EM tagging device. A moduleassociated with the radar system may identify a spectrally taggedcomponent of reflected signals received by the radar system from thetarget, the clutter source and the EM tagging device. The modulemonitors the spectrally tagged component of the reflected signalsreceived by the radar system, compensates for variations in an un-taggedcomponent of the reflected signals caused by the clutter source and EMtagging device, and subtracts contamination caused by the clutter sourceand EM tagging device and their interactions with the target, to providea radar cross section of the target with reflected signals from theclutter source removed. An output device may present the radar crosssection of the target with the reflected signals from the clutter sourceremoved.

In accordance with another embodiment, a system for performing radarcross section measurements of a target may include a first coherentsignal generator to generate a test signal at a selected test frequency.A first power divider may split the test signal into an un-modulatedtransmit test signal to be transmitted by the system to the target and afirst sample of the transmit test signal. A second power divider maysplit the first sample of the transmit test signal into a second sampleof the transmit test signal and a third sample of the transmit testsignal. The second sample of the transmit test signal may be a referencesignal for use in detecting un-modulated reflected signals received bythe system. The system may also include a second coherent signalgenerator to generate a tagging signal at the selected test frequencyshifted or offset by a chosen spectral tagging modulation frequency. Athird power divider may split the tagging signal into a first sampletagging signal and a second sample tagging signal. A mixer may beprovided to mix the first sample tagging signal and the third sample ofthe transmit test signal. An output signal from the mixer may be useableto drive an EM tagging device to spectrally tag a clutter source. Areceive path power divider may split reflected signals received by thesystem into a first sample of the received reflected signals and asecond sample of the received reflected signals. The reflected signalsreceived by the system include both un-modulated components from thetarget, clutter source and EM tagging device, and modulated componentsfrom the clutter source and EM tagging device only. A first coherentreceiver may detect a time-average of un-modulated components of thefirst sample received reflected signals using the second sample of thetransmit test signal as the reference signal, wherein the referencesignal is un-modulated. A second coherent receiver may be provided todetect a time-average of modulated components of the second samplereceived reflected signals using the second sample tagging signal. Amodule is provided to determine the radar cross section of the targetwith reflected signals from the clutter source removed by monitoring thetime-average of the modulated components of the received reflectedsignals, adjusting the time-average of a contribution of the cluttersource and EM tagging surface to the un-modulated components, andsubtracting these adjusted components of the received reflected signalsfrom the received reflected signals. An output device may present theradar cross section of the target with reflected signals from theclutter source and EM tagging device removed.

In accordance with another embodiment, a system for performing radarcross section measurements of a target may include a coherent signalgenerator to generate a test signal at a selected test frequency. Apower divider may be provided to split the test signal into anun-modulated transmit test signal to be transmitted by the system and asample of the transmit test signal. A radio frequency (RF) pulse formingswitch may form the un-modulated transmit test signal into pulses fortransmission to the target. A pulse generator may control timing of theRF pulse forming switch. A counter may receive pulse signals from thepulse generator to provide a modulated tagging signal that changes statewith each pulse at a predefined modulation frequency. The modulatedtagging signal is useable to toggle an EM tagging device between a firstradar cross section state and a second radar cross section state tospectrally tag a clutter source. A coherent receiver may be used todetect reflected signals received by the system. A signal processor mayprocess the reflected signals to provide a radar cross section of thetarget with reflected signals from the clutter source and EM taggingdevice removed. The system may also include an output device to presentthe radar cross section of the target with reflected signals from theclutter source and EM tagging device removed.

In accordance with another embodiment, a method for performing radarcross section measurements of a target may include transmitting apredetermined signal to the target and transmitting a spectral taggingsignal to an EM tagging device located proximate to a clutter source tospectrally tag the clutter source. The EM tagging device may spectrallytag the clutter source by causing changes in an electromagnetic signalreflected by the clutter source when the predetermined signal istransmitted to the target, the clutter source and the EM tagging device.The method may also include receiving reflected signals from the target,the clutter source and the EM tagging device. Contributions of theclutter source and the EM tagging device to an un-modulated component ofthe received reflected signals may be determined by monitoringvariations in a modulated component of the received reflected signals.The method may further include adjusting for the variations in theun-modulated component to remove contributions of the clutter source andthe EM tagging device and any interactions with the target from thereceived reflected signals to provide the radar cross section of thetarget without influence of the clutter source. The radar cross sectionof target may be presented without influence of the clutter source.

In accordance with another embodiment, a method for performing radarcross section measurements of a target may include transmitting testpulse signals at a selected pulse repetition frequency to the target.The method may also include generating tagging pulses to toggle aspectral tagging device between a first RCS state and a second RCS statewith each successive tagging pulse, wherein even numbered pulsescorrespond to the first RCS state and odd numbered pulses correspond tothe second RCS state. The spectral tagging device is placed proximate toa clutter source to spectrally tag the clutter source. The method mayalso include computing a sum of the even and odd numbered reflectedpulses to represent un-modulated reflected signals and computing adifference of the even and odd numbered reflected pulses to representmodulated reflected signals. The radar cross section of the targetwithout interference of the clutter source may be determined bymonitoring the modulated reflected signals to detect variations incontributions from the clutter source and EM tagging device to theun-modulated reflected signals. Adjusting for this variation allows theremoval of these contributions from the combined reflected signal.

In accordance with another embodiment, a method for performing radarcross section measurements of a target may include transmitting apredetermined signal at a selected frequency with only a clutter sourceand spectral tagging device in a RCS range. The method may also includedriving the spectral tagging device to cause a periodic time varyingchange of the spectral tagging device between a first RCS state and asecond RCS state at a chosen frequency to produce clutter sourcescattering at the selected frequency and a sideband frequency that is acombination of the selected frequency and the chosen frequency. Thescattered signals at the selected frequency and the sideband frequencymay be measured without the target in the RCS range. A ratio of thescattered signals, without the target in the RCS range, at the selectedfrequency to the scattered signals at the sideband frequency may bedetermined. The predetermined signal may again be transmitted at theselected frequency with the target in the RCS range and the EM taggingdevice may be driven to cause the periodic time varying change of thespectral tagging device between the first RCS state and the second RCSstate at the chosen frequency. Scattered signals may be measured at theselected frequency and the sideband frequency. Which scattered signalsare from the clutter source at the selected frequency may be determinedby multiplying the ratio of the scattered signals at the selectedfrequency without the target in the RCS range to the scattered signalsat the sideband frequency without the target in the RCS range times thescattered signals at the sideband frequency with the target in the RCSrange. A radar cross section of the target may be determined bysubtracting the scattered signals from the clutter source from anaverage scattered signal with the target in the RCS range at theselected frequency with the spectral tagging device being driven betweenthe first and second RCS states.

Other aspects and features of the present invention, as defined solelyby the claims, will become apparent to those ordinarily skilled in theart upon review of the following non-limited detailed description of theinvention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following detailed description of embodiments refers to theaccompanying drawings, which illustrate specific embodiments of theinvention. Other embodiments having different structures and operationsdo not depart from the scope of the present disclosure.

FIG. 1 is a block schematic diagram of an exemplary system for measuringa radar cross section of a target and removing clutter from the radarcross section measurement in accordance with an embodiment of thepresent disclosure.

FIG. 2 is a schematic diagram of an example of an electromagnetic (EM)tagging device or EM surface in accordance with an embodiment of thepresent disclosure.

FIG. 3 is a schematic diagram of an example of an EM tagging device orEM surface in accordance with another embodiment of the presentdisclosure.

FIG. 4 is a schematic diagram of an example of a radar cross sectionmeasurement device including a spectral tagging module in accordancewith an embodiment of the present disclosure.

FIG. 5 is a schematic diagram another example of a radar cross sectionmeasurement device including a spectral tagging module in accordancewith another embodiment of the present disclosure.

FIG. 6 is a flow chart of an example of a method for measuring a radarcross section of a target and removing clutter from the radar crosssection measurement in accordance with an embodiment of the presentdisclosure.

FIG. 7 is a flow chart of an example of a method for measuring a radarcross section of a target and removing clutter from the radar crosssection measurement in accordance with another embodiment of the presentdisclosure.

DETAILED DESCRIPTION

The following detailed description of embodiments refers to theaccompanying drawings, which illustrate specific embodiments of theinvention. Other embodiments having different structures and operationsdo not depart from the scope of the present invention.

FIG. 1 is a block schematic diagram of an exemplary system 100 formeasuring a radar cross section of a target 102 and removing clutterfrom the radar cross section measurement in accordance with anembodiment of the present disclosure. The system 100 may include a radarsystem 104 or RCS measurement system. The radar system 104 may include aRF or radar signal transmitter and receiver or combination transmitterand receiver 106. An antenna 108 may be associated with or coupled tothe radar system 104 to transmit signals 110 and receive reflectedsignals 112 and 114 reflected or scattered from the target 102 and anyclutter sources 116, respectively.

The system 100 may also include an EM tagging device 118. The EM taggingdevice 118 is locatable proximate to the clutter source 116 tospectrally tag the clutter source 116 by causing changes in theelectromagnetic signal 114 reflected by the clutter source 116 when apredetermined radar signal 110 transmitted by the radar system 104 viaantenna 108 is incident on the target 102, the clutter source 116 andthe EM tagging device 118.

The system 100 may also include a signal processor and/or module 120 todetect and remove clutter or reflected signals from the clutter source116. The module 120 may be associated with or integrally formed as partof the radar system 104 or RCS measurement system. The module 120 orsignal processor may identify a spectrally tagged component of thereflected signals 112 and 114 received by the radar system 104 from thetarget 102, the clutter source 116 and the EM tagging device 118. Thespectrally tagged component of the reflected signals 112 and 114 may beremoved or subtracted from a combined reflected signal received by theradar system 104, as described in more detail herein, to provide a radarcross section of the target 102 with the reflected signals from theclutter source 116 and EM tagging device 118 and any electromagneticinteraction between the clutter source 116 and the target 102 removed.Electromagnetic interaction or RF interaction between the combinedclutter source 116 and EM tagging device 118 and the target 102 mayinvolve any secondary or higher order reflected signals or EM fieldsreflected or scattered from the combined clutter source 116 and the EMtagging device 118 to the target 102 and reflected or scattered from thetarget 102 back to the antenna 108 and radar system 104, or alternatelyreflected or scattered from the target 102 to the combined cluttersource 116 and the EM tagging device 118 and back to the antenna 108.

The module 120 monitors the spectrally tagged component of the reflectedsignals or modulated return signals received by the radar system 100.The module 120 compensates for variations in un-modulated return signalsor an un-tagged component of the reflected signals caused by the cluttersource 116 and EM tagging device 118. Contamination caused by theclutter source 116 and EM tagging device 118 and their interactions withthe target 102 are removed from the reflected signal to provide a radarcross section of the target 102 with reflected signals from the cluttersource 116 and EM tagging device 118 and any electromagnetic interactionbetween the clutter source 116 and the target 102 removed.

An output device 122 may be associated with the radar system 104 topresent the radar cross section of the target 102 or a representation ofthe radar cross section of the target 102 with reflected or scatteredsignals from the clutter source 116 and tagging device 118 and anyinteraction between the clutter source 116 and target 102 removed. Theoutput device 122 may be a monitor or display, a printer or other devicecapable of presenting the RCS to a user.

The radar system 104 may also include a spectral tagging module 124. Thespectral tagging module may generate spectral tagging signals to drivethe EM tagging device 118 to cause the tagging device 118 to change ormodulate between radar cross section states to spectrally tag theclutter source 116 as described in more detail herein.

The EM tagging device 118 may be an EM surface or any device for whichthe radar cross section can be changed or modulated electronically.Examples of EM surfaces are illustrated in FIGS. 2 and 3. In the exampleof FIG. 2, the EM surface 200 consists of a plurality of conductive ormetallic elements 202 that are disposed in parallel on both sides of asubstrate 204 at a predetermined spacing from one another forming a gap206 between adjacent conductive elements 202 except for the twoconductive elements directly below a coax connector 210 and cable 212which form a single conductive element serving as the ground connectionfor the outer shield of the coax cable 212. The substrate 204 may besubstantially rectangular or square shaped and made from a dielectricmaterial. The elements 202 may also be substantially square orrectangular shaped with triangular shaped elements at the corners of thesubstrate 202 as illustrated in FIG. 2. The elements 202 may form a looparound the perimeter on both sides of the substrate 204 forming twoparallel conductive paths separated by parallel gaps 206 allowingimproved impedance matching to the coax connector 210 and cable 212.FIG. 2 shows the entry point of the coax cable 212 center conductor 214from below, through the substrate 204 and attached to two diodes thenceconnected to conductive elements 202 on either side of the entry pointor center conductor 214. Two holes 215 and conductors 216 carry theconnection from the two conductive elements 202 on either side of thetwo holes 215 and conductors 216 and thence to the bottom layer orunderside 217 of the substrate 204 and then connected to correspondingconductive elements 202 (not visible in FIG. 2) on the bottom layer orunderside 217 of the substrate 204. The elements 202 may beinterconnected across each gap 206 by a diode 208 or field effecttransistor (FET). The diodes 208 are so arranged that currents flow fromthe coax cable 212 center conductor 214 around either side of the EMsurface 200, through the holes 215 and conductors 216 and then reversingdirections on the lower surface or underside 217 of the substrate 204 tothe single conductive element serving as the ground connection for theouter shield of the coax cable 212. If the diodes 208 or FETs areforward biased, current can flow between the elements 202 and the EMsurface 200 may appear electrically as a continuous conductive loop. Ifthe diodes 208 or FETs are unbiased or are off, the EM surface 200 willappear electrically as a collection of short unconnected conductiveelements 202. Accordingly, the RCS will be significantly differentbetween the two cases or states.

In the example of FIG. 3, the EM surface 300 may be a straight line ofconductive elements 302 disposed on either side of a substrate 304 witha predetermined spacing between adjacent elements 302 forming a gap 306.A diode 308 or FET may interconnect each of the elements 302 on bothsides of substrate 304 with a “return” path to the lower or under side309 of the substrate 304 with oppositely directed diodes 310 andelements 312 on the under side 309. Similar to EM surface 200 a coaxcable 314 is used to feed thru a hole 316 in the substrate 304 from theunder side of 309 of the substrate 304 and “return” current to an upperside of 318 to a coax cable outer conductor 320. The diodes 308 and 310or FETs may be biased to appear electrically as one long wire segment ormay be unbiased to appear electrically as a linear collection of shortwire or conductive elements 302 and 312. Again the RCS of each of thesetwo states will be different.

In the examples of using diodes in EM surfaces 200 and 300, two distinctRCS states can be created. If FETs are used, multiple different RCSstates may be created as the resistance across the gaps 206 and 306 maybe continuously varied with the bias voltage. Further description of EMsurfaces may be found in Ruck, G. T. et Radar Cross Section Handbook,Vol. 1, 1970, Plenum Press, N.Y. pp. 289-290.

The EM surface 200 or 300 or EM tagging device 118 can “sense” changesin the local fields within a close proximity of the EM surface 200 or300 by the EM surface 200 or 300 having a variable RCS. The intensity ofthe reflected fields of the EM tagging device 118 or surface 200, 300 isa product of the RCS and the local fields. By modulating the RCS of theEM surface 200, 300 or EM tagging device 118, the intensity of thereflected fields from the vicinity of the EM tagging device 118 are alsomodulated. Reflected fields from other regions (without a modulated EMsurface or tagging device) are not modulated. Therefore, all reflectedsignals that contain the modulation can be determined to have come fromthe vicinity of the EM surface 200, 300 or tagging device 118 only, andnowhere else.

The absolute value of the local fields in the vicinity of the EM taggingdevice 118 is not important. What is critical is being able to determine“changes” in the local fields due to the insertion of an additionalscatterer or target. After the modulated and un-modulated components ofthe reflected signals are separated, the modulated components can beobserved for changes in reflected fields with and without the target 102present. These changes are proportional to the change in the localfields in the vicinity of the EM tagging device 118 due to the presenceof the target 102. Since the clutter source 116, by itself, has aconstant RCS, and since the reflected fields are proportional to theproduct of the RCS and the local fields, determining the changes in thelocal fields allows a determination of the changes in the reflectedfields from the clutter source 116. Accordingly, placing the EM taggingdevice 118 proximate to the clutter source 116, effectively “tags” thereflected fields or signals from the clutter source 116.

Employing proper design of the EM tagging device 118 and associatedclutter source 116, the reflected disturbance of the target fields areinsignificant. There will be some small perturbation due to un-tagged(un-modulated) reflections from the clutter source 116 in the directionof the target 102. However, this field or signal will, in general, beextremely small compared to the incident fields of the radar system 104or instrumentation radar, and can usually be considered negligible. EMtagging device 118 can be selected or designed to substantially minimizethe effect of any un-modulated reflected fields or signals.

Examples of a radar system or radar cross section measurement device 104including components or modules for spectral tagging, signal processingand detecting and removing clutter will be described with reference toFIGS. 4 and 5 below. In one embodiment of the system 100 similar to thatillustrated in FIG. 4, the radar system 104 may include a spectraltagging module 124 to generate a coherent sinusoidal modulated signal ata selected modulation frequency to drive the EM tagging device 118. Theselected frequency is offset from an un-modulated test frequency of thepredetermined radar signal 110 transmitted by the antenna 108. Aseparate receive channel offset from the test frequency by the selectedmodulation frequency separates modulated and un-modulated components ofthe received reflected signals to provide the radar cross section of thetarget with interference from the clutter source removed.

In another embodiment similar to that illustrated in FIG. 5, the radarsystem 104 may include a spectral tagging module 124 to generate asquare wave modulated signal at a selected pulse repetition frequencydifferent from a pulse repetition frequency of the predetermined radarsignal 110 to drive the EM tagging device 118. The EM tagging device 118may be toggled between two distinct radar cross section states with eachpulse of the square wave modulated signal. The signal processor 120separates modulated and un-modulated components of the receivedreflected signals to provide the radar cross section of the target withinterference from the clutter source removed. Even numbered pulses ofthe received reflected signals represent a first radar cross sectionstate of the EM tagging device 118 and odd numbered pulses of thereceived reflected signals represent a second radar cross section stateof the EM tagging device 118. A sum of the even and odd pulsesrepresents the un-modulated components of the received reflected signaland a difference of the even and odd pulse represents the modulatedcomponents of the received reflected signal.

FIG. 4 is a schematic diagram of an example of a radar cross sectionmeasurement device 400 including a spectral tagging module 402 inaccordance with an embodiment of the present disclosure. The RCSmeasurement device 400 may form part of or may be used for the radarsystem or RCS measurement system 104 in FIG. 1. The device 400 mayinclude a first coherent signal generator 404 to generate a test signalat a selected test frequency to be transmitted to a target 406 formeasuring the RCS of the target 406. The test signal may be a coherentsinusoidal signal. The test signal generated by generator 404 may besplit or divided into three paths by two power dividers or splitters 408and 410. The first power divider 408 may split the test signal into atransmit test signal in transmit path 412 and a first sample signal ofthe transmit test signal in circuit path 414. The transmit test signalmay be pulsed by a radio frequency (RF) switch 416 and amplified by anamplifier 418. A circulator 420 passes the pulsed transmit test signal422 to an antenna 424 for transmission of the pulsed, un-modulatedtransmit test signal 422 to the target 406 for measuring the RCS of thetarget 406.

The second power divider 410 or splitter splits the first sample of thetransmit test signal via circuit path 414 into a second sample of thetransmit test signal in circuit path 426 and a third sample of thetransmit test signal in circuit path 428. The second sample of thetransmit test signal may be used as a reference signal for basebandchannel detection. That is, the second sample of the transmit testsignal may be used as a reference for detecting un-modulated, reflectedsignals received by the radar system or RCS measurement device 400 in afirst coherent receiver 430. The first coherent receiver 430 may be anI/Q (quadrature) mixer or similar device.

The system 400 may also include a second coherent signal generator 432to generate a tagging signal at the selected test frequency shifted oroffset by a chosen spectral tagging modulation frequency. Accordingly,the second coherent generator 432 may generate a coherent sinusoidalmodulation signal for spectral tagging a clutter source that is offsetfrom the transmitted test signal. A third power divider 434 may splitthe tagging signal into a first sample tagging signal in circuit path436 and a second sample tagging signal in circuit path 438. A mixer 440may mix the first sample tagging signal and the third sample of thetransmit test signal. The system 400 may include a low pass filter (LPF)442 to filter an output from the mixer 440 to form a resulting signal.The resulting signal may be applied to an EM tagging device 444 fordriving the EM tagging device 444 to spectrally tag a clutter source446. The resulting signal may be a modulated signal at the selected testfrequency which is coherent with the un-modulated transmit test signal.

The reflected signals 448 and 450 from the target 406 and clutter source446 and received by the antenna 424 include both modulated andun-modulated components. The reflected signals 448 and 450 pass throughthe circulator 420 and are amplified by the amplifier 452 in a receivepath 453. A RF switch 454 coupled to the amplifier 452 may range gate ortime gate the received reflected signals 448 and 450 afteramplification. The receive signal is time-gated by RF switch 454 toisolate the very large transmitter leakage coupling through thecirculator 420 from the very weak reflections or reflected signals 448from the target 406. The RF switch 454 is open during the time thesystem 400 is transmitting, preventing this large signal in the path 453from entering the receive components 462 or 430. The RF switch 454 isthen closed after sufficient time has passed for the EM energy to travelto the target 406 and back, allowing the target reflections to bedetected by the first coherent receiver 430.

A receive path power divider 456 or splitter may split the reflectedsignals received by the RCS measurement system 400 or radar system intoa first sample of the received reflected signals in receive circuit path458 and a second sample of the received reflected signals in receivecircuit path 460. The first coherent receiver 430 may detect atime-average of the un-modulated components of the received reflectedsignals using the second sample of the transmit test signal via circuitpath 426 as the reference signal, wherein the reference signal isun-modulated. The first coherent receiver 430 may be an I/Q (quadrature)mixer or similar device for detecting a time-average of the un-modulatedcomponents of the received reflected signals.

The system 400 may include a second coherent receiver 462 to detect atime-average of the modulated components of the received reflectedsignals using the second sample tagging signal via circuit path 438 as areference. The second coherent receiver 462 may be an I/Q (quadrature)mixer or similar device. The RCS of the target 406 with cluttercontamination removed may be determined from the detected un-modulatedcomponents of the first coherent receiver 430 and modulated componentsof the second coherent receiver 462 of the received reflected signals448 and 450 both with and without the target 406 in the RCS range 407.Which scattered signals 448 and 450 are from the clutter source 446 andEM tagging device 444 at the selected frequency may be determined bymultiplying the ratio of the scattered signals at the selected frequencywithout the target 406 in the RCS range 407 to the scattered signals atthe sideband frequency without the target 406 in the RCS range 407 timesthe scattered signals at the sideband frequency with the target 406 inthe RCS range 407. A radar cross section of the target 406 may then bedetermined by subtracting the scattered signals from the clutter source446 (described above) from an average scattered signal, driven at theselected frequency with the EM tagging device 444 between the first andsecond RCS states, with the target 406 in the RCS range 407.

The second coherent signal generator 432, mixer 440, LPF 442 and secondcoherent receiver 462 may define or form at least part of the spectraltagging module 402.

FIG. 5 is a schematic diagram of another example of a radar crosssection measurement device 500 including a spectral tagging module 502in accordance with another embodiment of the present disclosure. Thedevice 500 may include a coherent signal generator 504 to generate atest signal at a selected test frequency. A power divider 506 may splitthe test signal into an un-modulated transmit test signal and a sampleof the transmit test signal. A RF pulse forming switch 508 coupled tothe power divider 506 may form the transmit test signal into pulses fortransmission by an antenna 510 to a target 512. The transmit test signalpulses may be amplified by an amplifier 514 and passed through acirculator 516 to the antenna 510.

The RCS measurement device 500 or radar system may also include a pulsegenerator 518 to generate timing pulses to control timing of the RFpulse forming switch 508. A sample of the timing pulses sent to the RFpulse forming switch 508 are sent to a counter 520 or similar devicefrom the pulse generator 518 to provide modulated tagging signals thatchange state with each pulse at a predefined modulation frequency. Thecounter 520 may be a divide-by-two counter to provide the modulatedtagging signals that change state with each pulse. The modulated taggingsignal is useable to toggle an EM tagging device 522 or EM surface,similar to that previously described, between a first RCS state andsecond RCS state with each pulse at a predefined modulation rate tospectrally tag a clutter source 524. Accordingly, the EM tagging device522 may be toggled between two distinct states using a square-wavesignal pattern from the pulse generator 518 and divide-by-two counter520. The frequency of the square wave modulation may be ½ the pulserepetition frequency of the transmitted test signal to the target 512.The frequency of the square wave modulation may also be ½N of the pulserepetition frequency, where N may be an integer greater than or equalto 1. For example, ¼, ⅙, or higher divisions of the pulse repetitionfrequency could also be used. For these higher order wave modulations,the pulses must be divided into even and odd groups of N pulses.

Reflected signals received by the antenna 510 will pass through thecirculator 516 and may be amplified by another amplifier 526 in thereceive path of the RCS measurement device 500. The received reflectedsignal may be gated by an RF switch 528. The gated signal from the RFswitch 528 may be detected by a coherent receiver 530, I/Q (quadrature)mixer or similar device using a sample of the transmit test signal fromthe power divider 506. The signal detected by the I/Q (quadrature) mixer530 may be processed by a signal processor 532 to provide the radarcross section of the target 512 with any clutter contamination removed.

The signal processor 532 may be a digital signal processor (DSP). Ananalog-to-digital (A/D) converter 534 may receive the detected,reflected signals from the I/Q (quadrature) mixer 530 and may convertthe reflected signals to digital pulse signals. The DSP 532 processesthe digital pulse signals from the A/D converter 534. Even numberedpulse signals may correspond to the EM tagging device 522 in a firstradar cross section state and the odd numbered pulse signals maycorrespond to the EM tagging device 522 in the second radar crosssection state. The DSP 532 may compute a sum of the even and oddnumbered pulse signals to provide a sum term containing onlyun-modulated components of the received reflected signals. The DSP 532may compute a difference of the even and odd numbered pulse signals toprovide a difference term containing only modulated components of thereceived reflected signals. The radar cross section of the target 512with the reflected signals from the clutter source 524 and EM taggingdevice 522 removed may be determined by removing or subtracting theirun-modulated clutter contributions to the received reflected signalsfrom the combined or total received reflected signal. The un-modulatedcontributions of the clutter source 524 and EM tagging device 522 aredetermined by monitoring the variations in the modulated components ofthe received reflected signals. Variations in the un-modulatedcomponents of the received reflected signals may be adjusted orcompensated and removed or subtracted from the reflected signals toprovide the radar cross section of the target 524, as described in moredetail with reference to FIG. 7.

FIG. 6 is a flow chart of an example of a method 600 for measuring aradar cross section of a target and removing clutter from the radarcross section measurement in accordance with an embodiment of thepresent disclosure. The method 600 may be embodied in the system 100 ofFIG. 1 or performed by the system 100. In block 602, an EM taggingdevice or EM surface may be positioned proximate to any clutter sourcesto cause spectral tagging of the clutter sources. Similar to thatdescribed herein, a clutter source may be spectrally tagged by an EMtagging device in that any changes in the EM tagging device scatteringcauses changes in reflected signals or scattering from the associatedclutter source. The EM tagging device is placed relative to or in suchproximity of the associated clutter source to cause the spectraltagging.

In block 604, a predetermined signal may be transmitted from a RCSmeasurement system or radar system, such as the system 100, to a targetfor measuring the RCS of the target. The predetermined signal may be anun-modulated signal. A spectral tagging signal may also be transmittedto the EM tagging device. The spectral tagging signal may be modulatedto modulate or cause changes to the RCS of the EM tagging to causemodulation of reflected EM fields or signals from the EM tagging devicewhich in turn causes modulation or changes in the reflected fields orsignals from the clutter source.

In block 606, reflected or scattered EM fields or signals may bereceived by the system. The signal strength, amplitude and phase of thereflected signals or other parameters characterizing the reflected orscattered EM fields or signals may be measured to determine or measurethe RCS of the target.

In block 608, the modulated and un-modulated components of the receivedsignals or combined received signals may be separated to identify thespectrally tagged components or portions of the reflected or scatteredsignals or fields.

In block 610, the contribution of the clutter source and EM taggingdevice to the un-modulated received signals may be determined bymonitoring the variations in the modulated receive signals. Variationsin the un-modulated received signals or un-tagged component of thereflected signals caused by the clutter source and EM tagging device maybe compensated or adjusted to remove their contributions of the cluttersource and EM tagging device from the combined reflected signal or fieldto provide the reflected signal or field from the target with reflectedor scattered signals or fields from the clutter source and EM taggingdevice and substantially any interactions between the clutter source andtarget removed. The resulting signal or field will correspond to the RCSof the target.

In block 612, the RCS of the target may be presented on an output deviceof the system. The output device may be a monitor or display, a printeror other device capable of presenting the RCS to a user. The RCS may bea representation of the reflected or scattered signal or field from thetarget substantially without an influence or contamination of theclutter source.

FIG. 7 is a flow chart of an example of a method 700 for measuring aradar cross section of a target and removing clutter from the radarcross section measurement in accordance with another embodiment of thepresent disclosure. The method 700 may be embodied in the system 100 ofFIG. 1 or may be performed by the system 100.

In block 702, an EM tagging device, similar to EM tagging device 118 inFIG. 1, may be positioned proximate to any clutter source in the RCSrange to cause spectral tagging of the clutter source as describedherein, wherein changes in the EM tagging device scattering or reflectedsignals causes changes in the scattering or reflected signals from theclutter source.

In block 704, a predetermined test signal at a selected frequency (f₀)may be transmitted with only the clutter source and EM tagging devicepresent in the RCS range (target absent). The EM tagging device is alsodriven or activated by a predefined spectral tagging signal at a chosenspectral tagging frequency (Δf) to cause a periodic time varying changeof the RCS characteristics of the EM tagging device between two RCSstates. The time varying change may be a change in the impedancecharacteristics of the EM tagging device to cause the change in RCSstates. The predefined tagging signal driving the EM tagging deviceproduces, from the clutter source, scattering or reflected signals orfields, at a frequency of the incident field or predetermined signal(f₀) and at a sideband frequency. The sideband frequency is a functionof the frequency of the incident frequency or transmitted test frequencyand the spectral tagging frequency (f₁=f₀±Δf). As previously described,the EM tagging device may be an EM surface, such as the examplesdescribed with reference to FIGS. 2 and 3 or some other configurationdepending upon the RCS measurement range or environment and the natureof the clutter source. As previously described, the EM tagging device isplaced or positioned relative to the clutter source or within apredetermined proximity of the clutter source to cause the changes inthe scattered or reflected fields of the scatter source in response tothe changes or modulation of the RCS of the EM tagging device.

In block 706, an amplitude and phase of the scattered or reflectedfields or signals at frequencies f₀ and f₁ without the target presentmay be measured. A ratio of the signal strength or power of scatteredfields at f₀ to scattered fields at f₁ may be determined or calculated(E_(f0)/E_(f1)).

In block 708, the predetermined test signal at the selected frequency(f₀) may be transmitted with the target present in the RCS range. The EMtagging device may also be driven or activated by the predefined taggingsignal at the chosen spectral tagging frequency (Δf) to cause periodictime varying changes in the EM tagging device between at least the twoRCS states.

In block 710, the amplitude and phase of the scattered fields or signalsat the selected frequency or incident frequency f₀ and the sidebandfrequency (f₁) with the target present may be measured. The scatteredfields or signals from the clutter source at f₀ with the target present(E_(f0) ^(Clutterint)) may be determined by multiplying the ratiodetermined in block 706 of the scattered fields at f₀ to the scatteredfields at f₁ without the target present times the scattered field orsignal with the target present at the sideband frequency f₁ which isrepresented by Equation 1:E _(f0) ^(Clutterint) =E _(f1) ^(Tint)(E _(f0) /E _(f1))  Equation 1Where E_(f0) ^(Clutterint) is the scattered fields or signals from theclutter source or the clutter interference in the presence of targetinteraction at the incident or test frequency f₀. E_(f1) ^(Tint) is thescattered field or signal with the target present at the sidebandfrequency f₁.

In block 712, the scattered fields or signals of the target (E_(f0)^(T)) at the incident or transmitted test frequency f₀ may be determinedby subtracting the interacting clutter field (E_(f0) ^(Clutterint)) fromthe average scattered fields or signals (E_(f0) ^(Tint)) with the targetpresent at frequency f₀ with the EM tagging device being driven betweenRCS states to cause a periodic time varying change between the RCSstates. This calculation is represented by equation 2:E _(f0) ^(T) =E _(f0) ^(Tint) −E _(f0) ^(Clutterint)  Equation 2

In block 714, the RCS of the target at the selected incident frequencyor transmitted test frequency f₀ (RCS_(f0) ^(T)) with the clutterinterference removed corresponds to E_(f0) ^(T) in block 712:RCS_(f0) ^(T)=RCS_(Known) ^(Thry) ×|E _(f0) ^(T) /E _(Known)|²Where RCS_(Known) ^(Thry) is the theoretical RCS value of a known target(typically a sphere) and E_(Known) is the measured field or signal fromthe known target at frequency f₀ in the RCS range.

In block 716, the radar cross section of the target may be presented onan output device of an RCS measurement system or radar system, such asthe system 100 in FIG. 1 without scattering or influence from theclutter source

Although specific embodiments have been illustrated and describedherein, those of ordinary skill in the art appreciate that anyarrangement which is calculated to achieve the same purpose may besubstituted for the specific embodiments shown and that the embodimentsherein have other applications in other environments. This applicationis intended to cover any adaptations or variations of the presentdisclosure. The following claims are in no way intended to limit thescope of the disclosure to the specific embodiments described herein.

1. A system for performing radar cross section measurements of a target,comprising a radar system; an antenna associated with the radar systemto transmit signals and to receive reflected signals from the target anda clutter source; an EM tagging device locatable proximate to theclutter source to spectrally tag the clutter source by causing changesin an electromagnetic signal reflected by the clutter source when apredetermined radar signal transmitted by the radar system is incidenton the target, the clutter source and the EM tagging device; a moduleassociated with the radar system to identify a spectrally taggedcomponent of reflected signals received by the radar system from thetarget, the clutter source and the EM tagging device, wherein the modulemonitors the spectrally tagged component of the reflected signalsreceived by the radar system, compensates for variations in an un-taggedcomponent of the reflected signals caused by the clutter source and EMtagging device, and subtracts contamination caused by the clutter sourceand EM tagging device and their interactions with the target, to providea radar cross section of the target with reflected signals from theclutter source and EM tagging device and any electromagnetic interactionbetween the clutter source and the target removed; and an output deviceto present the radar cross section of the target with reflected signalsfrom the clutter source and EM tagging device and any electromagneticinteraction between the clutter source and the target removed.
 2. Thesystem of claim 1, wherein the EM tagging device comprises a changeableradar cross section.
 3. The system of claim 2, wherein the EM taggingdevice comprises an EM surface capable of being changed or modulatedbetween radar cross section states to spectrally tag the clutter source.4. The system of claim 3, wherein modulation of the radar cross sectionof the EM surface causes an intensity of reflected signals from theclutter source to also be modulated in response to the EM surface beingwithin a predetermined proximity of the clutter source.
 5. The system ofclaim 3, further comprising a component to modulate or change a radarcross section of the EM surface to cause modulation of reflected EMsignals from the EM surface, the EM surface being located proximate tothe clutter source so that modulation of the reflected EM signals fromthe EM surface causes modulation of reflected signals from the cluttersource.
 6. The system of claim 3, further comprising a component tocause a predetermined periodic time varying change of the EM surface tomodulate the EM surface between two or more radar cross section statesto spectrally tag the clutter source.
 7. The system of claim 6, whereinthe component comprises means to change the impedance characteristics ofthe EM surface to modulate the EM surface between the two or more radarcross section states.
 8. The system of claim 1, further comprising: aspectral tagging module to generate a coherent sinusoidal modulatedsignal at a selected modulation frequency to drive the EM taggingdevice, wherein the selected frequency is offset from an un-modulatedtest frequency of the predetermined radar signal; a separate receivechannel offset from the test frequency by the selected modulationfrequency to separate modulated and un-modulated components of thereceived reflected signals to provide the radar cross section of thetarget with interference from the clutter source removed.
 9. The systemof claim 1, further comprising: a spectral tagging module to generate asquare wave modulated signal at a selected pulse repetition frequencydifferent from a pulse repetition frequency of the predetermined radarsignal to drive the EM tagging device, wherein the EM tagging device istoggled between two distinct radar cross section states with each pulseof the square wave modulated signal; and a signal processor to separatemodulated and un-modulated components of the received reflected signalsto provide the radar cross section of the target with interference fromthe clutter source removed, wherein even numbered pulses of the receivedreflected signals represent a first radar cross section state of the EMtagging device and odd numbered pulses of the received reflected signalsrepresent a second radar cross section state of the EM tagging device,and wherein a sum of the even and odd pulses represents the un-modulatedcomponents of the received reflected signal and a difference of the evenand odd pulses represents the modulated components of the receivedreflected signal.
 10. The system of claim 1, wherein the radar systemcomprises: a first coherent signal generator to generate a test signalat a selected test frequency; a first power divider to split the testsignal into an un-modulated transmit test signal to be transmitted tothe target and a first sample of the transmit test signal; a secondpower divider to split the first sample of the transmit test signal intoa second sample of the transmit test signal and a third sample of thetransmit test signal, wherein the second sample of the transmit testsignal becomes a reference signal for use in detecting un-modulatedreflected signals received by the radar system; a second coherent signalgenerator to generate a tagging signal at the selected test frequencyshifted by a chosen spectral tagging modulation frequency; a third powerdivider to split the tagging signal into a first sample tagging signaland a second sample tagging signal; a mixer to mix the first sampletagging signal and the third sample of the transmit test signal; a lowpass filter to filter an output signal from the mixer to form aresulting signal, the resulting signal being applied to the EM taggingdevice for driving the EM tagging device to spectrally tag the cluttersource, wherein the resulting signal is a modulation signal at theselected test frequency which is coherent with the un-modulated transmittest signal; a receive path power divider to split the reflected signalsreceived by the radar system into a first sample of the receivedreflected signals and a second sample of the received reflected signals,wherein the reflected signals received by the radar system include bothun-modulated components and modulated components; a first coherentreceiver to detect a time-average of un-modulated components of thereceived reflected signals using the second sample of the transmit testsignal as the reference signal, wherein the reference signal isun-modulated; and a second coherent receiver to detect a time-average ofmodulated components of the received reflected signals using the secondsample tagging signal, wherein the radar cross section of the targetwith the clutter source and EM tagging device reflected signals removedis determined by monitoring variations in the modulated components ofthe received reflected signals, adjusting for these variations caused bya contribution of the clutter source and EM tagging device to theun-modulated components of the received reflected signals, andsubtracting an adjusted contribution of the clutter source and EMtagging device from the received reflected signals.
 11. The system ofclaim 10, wherein the radar system further comprises a circulator toalternately transmit the un-modulated transmit test signal and toreceive the reflected signals.
 12. The system of claim 1, wherein theradar system comprises: a coherent signal generator to generate a testsignal at a selected test frequency; a power divider to split the testsignal into an un-modulated transmit test signal to be transmitted by anantenna and a sample of the transmit test signal; a RF pulse formingswitch to form the un-modulated transmit test signal into pulses fortransmission from the antenna; a pulse generator to control timing ofthe RF pulse forming switch; a counter to receive pulse signals from thepulse generator to provide a modulated tagging signal that changes stateat a predefined modulation frequency, wherein the modulated taggingsignal is useable to toggle the EM tagging device between a first radarcross section state and a second radar cross section state to spectrallytag the clutter source; a coherent receiver to detect the reflectedsignals received by the radar system; and a signal processor to processthe reflected signals to provide the radar cross section of the target.13. The system of claim 12, further comprising an analog-to-digitalconverter to convert the reflected signals from the coherent receiver todigital pulse signals, and wherein the signal processor comprises adigital signal processor to process the digital pulse signals, whereineven numbered pulse signals correspond to the EM tagging device in thefirst radar cross section state and the odd numbered pulse signalscorrespond to the EM tagging device in the second radar cross sectionstate, and wherein the digital processor computes a sum of the even andodd numbered pulse signals to provide a sum term containing onlyun-modulated components of the received reflected signals and computes adifference of the even and odd numbered pulse signals to provide adifference term containing only modulated components of the receivedreflected signals, the radar cross section of the target with reflectedsignals from the clutter source and EM tagging device removed beingdetermined by monitoring the modulated components from the receivedreflected signals, adjusting the un-modulated components of the receivedreflected signals for clutter source and EM tagging device contributionsand subtracting the adjusted components.
 14. A system for performingradar cross section measurements of a target, comprising: a firstcoherent signal generator to generate a test signal at a selected testfrequency; a first power divider to split the test signal into anun-modulated transmit test signal to be transmitted by the system to thetarget and a first sample of the transmit test signal; a second powerdivider to split the first sample of the transmit test signal into asecond sample of the transmit test signal and a third sample of thetransmit test signal, wherein the second sample of the transmit testsignal is a reference signal for use in detecting un-modulated reflectedsignals received by the system; a second coherent signal generator togenerate a tagging signal at the selected test frequency shifted by achosen spectral tagging modulation frequency; a third power divider tosplit the tagging signal into a first sample tagging signal and a secondsample tagging signal; a mixer to mix the first sample tagging signaland the third sample of the transmit test signal, an output signal fromthe mixer being useable to drive an EM tagging device to spectrally taga clutter source; a receive path power divider to split reflectedsignals received by the system into a first sample of the receivedreflected signals and a second sample of the received reflected signals,wherein the reflected signals received by the system include bothun-modulated components and modulated components; a first coherentreceiver to detect a time-average of un-modulated components of thereceived reflected signals using the second sample of the transmit testsignal as the reference signal, wherein the reference signal isun-modulated; a second coherent receiver to detect a time-average ofmodulated components of the received reflected signals using the secondsample tagging signal; a module to determine the radar cross section ofthe target with reflected signals from the clutter source removed bymonitoring the time-average of the modulated components of the receivedreflected signals, adjusting the time-average of a contribution of theclutter source and EM tagging surface to the un-modulated components,and subtracting these adjusted components of the received reflectedsignals from the received reflected signals; and an output device topresent the radar cross section of the target with reflected signalsfrom the clutter source and EM tagging device removed.
 15. The system ofclaim 14, further comprising a low pass filter to filter the outputsignal from the mixer to form a resulting signal, the resulting signalbeing applied to the EM tagging device for driving the EM tagging deviceto spectrally tag the clutter source, wherein the resulting signal is amodulation signal at the selected test frequency which is coherent withthe un-modulated transmit test signal.
 16. The system of claim 14,further comprising: a first RF switch to pulse the un-modulated transmittest signal; a circulator to alternately transmit the pulsedun-modulated transmit test signal and to receive the reflected signals;and a second RF switch to gate the reflected signals.
 17. A system forperforming radar cross section measurements of a target, comprising acoherent signal generator to generate a test signal at a selected testfrequency; a power divider to split the test signal into an un-modulatedtransmit test signal to be transmitted by the antenna and a sample ofthe transmit test signal; a RF pulse forming switch to form theun-modulated transmit test signal into pulses for transmission to thetarget; a pulse generator to control timing of the RF pulse formingswitch; a counter to receive pulse signals from the pulse generator toprovide a modulated tagging signal that changes state at a predefinedmodulation frequency, wherein the modulated tagging signal is useable totoggle an EM tagging device between a first radar cross section stateand a second radar cross section state to spectrally tag a cluttersource; a coherent receiver to detect reflected signals received by thesystem; a signal processor to process the reflected signals to provide aradar cross section of the target with reflected signals from theclutter source and EM tagging device removed; and an output device topresent the radar cross section of the target with reflected signalsfrom the clutter source and EM tagging device removed.
 18. The system ofclaim 17, further comprising an analog-to-digital converter to convertthe reflected signals from the coherent receiver to digital pulsesignals, and wherein the signal processor comprises a digital signalprocessor to process the digital pulse signals, wherein even numberedpulse signals correspond to the EM tagging device in the first radarcross section state and the odd numbered pulses correspond to the EMtagging device in the second radar cross section state, and wherein thedigital processor computes a sum of the even and odd numbered pulses toprovide a sum term containing only un-modulated components of thereceived reflected signals and computes a difference of the even and oddnumbered pulses to provide a difference term containing only modulatedcomponents of the received reflected signals to provide the radar crosssection of the target with reflected signals from the clutter sourceremoved.
 19. A method for performing radar cross section measurements ofa target, comprising: transmitting a predetermined signal to the target;transmitting a spectral tagging signal to an EM tagging device locatedproximate to a clutter source to spectrally tag the clutter source bycausing changes in an electromagnetic signal reflected by the cluttersource when the predetermined signal is transmitted to the target, theclutter source and the EM tagging device; receiving reflected signalsfrom the target, the clutter source and the EM tagging device;determining contributions of the clutter source and the EM taggingdevice to an un-modulated component of the received reflected signals bymonitoring variations in a modulated component of the received reflectedsignals; adjusting for the variations in the un-modulated component toremove contributions of the clutter source and the EM tagging device andany interactions with the target from the received reflected signals toprovide the radar cross section of the target without influence of theclutter source; and presenting the radar cross section of target withoutinfluence of the clutter source.
 20. The method of claim 19, whereintransmitting the predetermined signal to the target comprisestransmitting an un-modulated signal to the target and whereintransmitting a spectral tagging signal to the EM tagging devicecomprises transmitting a modulated tagging signal to cause the EMtagging device to change or modulate between radar cross section statesto spectrally tag the clutter source.
 21. The method of claim 20,wherein monitoring variations in the modulated component of thereflected signals comprises separating modulated and un-modulatedcomponents in the reflected signals.
 22. The method of claim 20, whereinmonitoring the variations in the modulated component of the reflectedsignals allows compensation of variations in the un-modulated componentof the reflected signal due to the clutter source and EM tagging surfacefor an impact of interactions with the target, and subtracting orremoving the adjusted un-modulated clutter source, EM tagging surfaceand interaction contributions from the reflected signals.
 23. The methodof claim 19, wherein the EM tagging device is an EM surface and whereinthe method further comprises applying the spectral tagging signal to theEM surface to cause a predetermined periodic time varying change to theEM surface to modulate the EM surface between a first radar crosssection state and a second radar cross section state to spectrally tagthe clutter source.
 24. A method for performing radar cross sectionmeasurements of a target, comprising: transmitting test pulse signals ata selected pulse repetition frequency to the target; generating taggingpulses to toggle an EM tagging device between a first RCS state and asecond RCS state with each successive tagging pulse, wherein evennumbered pulses correspond to the first RCS state and odd numberedpulses correspond to the second RCS state, wherein the EM tagging deviceis placed proximate to a clutter source to spectrally tag the cluttersource; computing a sum of the even and odd numbered reflected pulses torepresent un-modulated reflected signals; computing a difference of theeven and odd numbered reflected pulses to represent modulated reflectedsignals; and determining the radar cross section of the target withoutinterference of the clutter source by monitoring the modulated reflectedsignals; adjusting the un-modulated contributions from the cluttersource and EM tagging device; and subtracting the adjusted un-modulatedcontributions from a combined reflected signal.
 25. The method of claim24, wherein the tagging pulses are modulated at one half the pulserepetition frequency of the test pulse signals.
 26. A method forperforming radar cross section measurements of a target, comprising:transmitting a predetermined signal at a selected frequency with only aclutter source and an EM tagging device in a RCS range; driving the EMtagging device to cause a periodic time varying change of the EM taggingdevice between a first RCS state and a second RCS state at a chosenfrequency to produce clutter source scattering at the selected frequencyand a sideband frequency that is a combination of the selected frequencyand the chosen frequency; measuring scattered signals at the selectedfrequency and the sideband frequency without the target in the RCSrange; determining a ratio of the scattered signals at the selectedfrequency to the scattered signals at the sideband frequency;transmitting the predetermined signal at the selected frequency with thetarget in the RCS range; driving the EM tagging device to cause theperiodic time varying change of the spectral tagging device between thefirst RCS state and the second RCS state at the chosen frequency;measuring scattered signals at the selected frequency and the sidebandfrequency; determining which scattered signals are from the cluttersource at the selected frequency by multiplying the ratio of thescattered signals at the selected frequency to the scattered signals atthe sideband frequency times the scattered signals at the sidebandfrequency with the target in the RCS range; determining a radar crosssection of the target by subtracting the scattered signals from theclutter source from an average scattered signal with the target in theRCS range at the selected frequency with the EM tagging device beingdriven between the first and second RCS states; presenting the radarcross section of the target.